1. Field of the Invention
The present invention relates to a vehicle-driving power transmission provided with a hydrostatic stepless transmission (hereinafter, “HST”). Especially, the present invention relates to an integral transmission, which is defined as integrally including an HST and a forward/backward traveling direction selecting unit (reverser) in this application.
2. Background Art
As disclosed by JP H10-167687 A, there is a conventional forklift equipped with an HST serving as a transmission for transmitting power of an engine serving as a prime mover to drive wheels. In this forklift, a hydraulic pump is directly attached to the engine, a hydraulic motor is directly attached to a transaxle incorporating a differential mechanism, and the hydraulic pump and motor are fluidly connected to each other via hydraulic fluid pipes so as to constitute the HST. This HST is not easily assembled because the hydraulic pump and the hydraulic motor are attached to different places separately from each other.
In this regard, there is a conventional HST whose hydraulic pump and motor are assembled integrally as disclosed by JP 2003-14079 A. In this HST, a cylinder block is fixed on a pump shaft, pump plungers and motor plungers are fitted into respective pump and motor cylinder holes bored in the cylinder block so as to be reciprocally movable in parallel to the pump shaft, a pair of fluid passages are formed in the cylinder block so as to be interposed between the pump cylinder holes fitting the pump plungers therein and the motor cylinder holes fitting the motor plungers therein, a movable swash plate serving as a pump swash plate is pressed against heads of the pump plungers projecting outward from one end surface of the cylinder block, and a motor swash plate is formed on a motor shaft member relatively rotatably fitted on the pump shaft and is pressed against heads of the motor plungers projecting outward from the other end surface of the cylinder block, so that the hydraulic pump and motor are made to be coaxial to each other.
Further, the cylinder block is provided therein with spool valves connected to the pump cylinder holes and with spool valves connected to the motor cylinder holes. The spool valves change their slide positions in correspondence to tilting movement of the pump swash plate, thereby changing a flow of hydraulic fluid between higher and lower pressurized fluid passages serving as the pair of fluid passages. The rotary speed of the motor shaft member having the motor swash plate pressed against the motor plungers changes according to change of the flow of hydraulic fluid. More specifically, when the tilt angle of the pump swash plate is zero, i.e., when the pump swash plate is disposed at its neutral position, the rotary speed of the motor shaft member is equal to the rotary speed of the cylinder block (and the pump shaft). The rotary speed of the motor shaft member relative to the cylinder block increases as the pump swash plate is tilted from the neutral position in one direction, and the rotary speed of the motor shaft member relative to the cylinder block decreases as the pump swash plate is tilted from the neutral position in the other direction.
The HST is radially compact with respect to the pump shaft due to the coaxial arrangement of the hydraulic pump and motor. Further, the HST is axially minimized due to the cylinder block which is common to both the hydraulic pump and motor.
This HST is now compared with a typical HST in which hydraulic pump and motor have respective cylinder blocks and a motor shaft serving as an HST output member is fixed to the cylinder block of the hydraulic motor. As for the typical HST, the hydraulic motor is rotated in one direction by tilting a movable swash plate serving as a pump swash plate in one direction from its neutral position, and the hydraulic motor is rotated in the opposite direction by tilting the movable swash plate in the opposite direction from the neutral position. On the contrary, as for the above-mentioned HST having the common cylinder block for the hydraulic pump and motor, the rotary speed of the motor shaft member becomes zero when the pump swash plate reaches a maximum tilt angle in one direction from the neutral position, and the rotary speed of the motor shaft member becomes its maximum when the pump swash plate reaches a maximum tilt angle in the opposite direction from the neutral position, so that the rotational direction of the motor shaft member is constant while the pump swash plate moves between the maximum tilt angle in one direction from the neutral position and the maximum tilt angle in the opposite direction from the neutral position. In other words, as for the HST whose hydraulic pump and motor have respective cylinder blocks, the tilt of the pump swash plate from the neutral position to the maximum tilt angle in one direction provides the variable speed range of output rotation of the HST in one direction, and the tilt of the pump swash plate from the neutral position to the maximum tilt angle in the opposite direction provides the variable speed range of output rotation of the HST in the opposite direction. On the contrary, as for the HST whose hydraulic pump and motor have a common cylinder block, the whole tilt range of the pump swash plate between the maximum tilt angle in one direction from the neutral position and the maximum tilt angle in the opposite direction from the neutral position provides the whole variable speed range of output rotation of the HST in only one direction. Therefore, the HST having a common cylinder block is adapted to be combined with a forward/backward traveling direction selecting device, i.e., reverser, which includes a forward traveling gear train, a backward traveling gear train and a reverser clutch for selecting either the forward or backward traveling gear train, so as to ensure a wide variable speed range of a vehicle in each of forward and backward traveling directions of the vehicle.